THORACIC VISCERA

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THORACIC VISCERA




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Thoracic Cavity


The thoracic cavity is bounded by the walls of the thorax and extends from the superior thoracic aperture, where structures enter the thorax, to the inferior thoracic aperture. The diaphragm separates the thoracic cavity from the abdominal cavity. The anatomic structures that pass from the thorax to the abdomen go through openings in the diaphragm (Fig. 10-2).



The thoracic cavity contains the lungs and heart; organs of the respiratory, cardiovascular, and lymphatic systems; the inferior portion of the esophagus; and the thymus gland. Within the cavity are three separate chambers: a single pericardial cavity and the right and left pleural cavities. These cavities are lined by shiny, slippery, and delicate serous membranes. The space between the two pleural cavities is called the mediastinum. This area contains all the thoracic structures except the lungs and pleurae.



Respiratory System


The respiratory system consists of the pharynx (described in Chapter 15 in Volume 2), trachea, bronchi, and two lungs. The air passages of these organs communicate with the exterior through the pharynx, mouth, and nose, each of which, in addition to other described functions, is considered a part of the respiratory system.



TRACHEA


The trachea is a fibrous, muscular tube with 16 to 20 C-shaped cartilaginous rings embedded in its walls for greater rigidity (Fig. 10-3, A). It measures approximately ½ inch (1.3 cm) in diameter and 4½ inches (11 cm) in length, and its posterior aspect is flat. The cartilaginous rings are incomplete posteriorly and extend around the anterior two thirds of the tube. The trachea lies in the midline of the body, anterior to the esophagus in the neck. In the thorax, the trachea is shifted slightly to the right of the midline as a result of the arching of the aorta. The trachea follows the curve of the vertebral column and extends from its junction with the larynx at the level of the sixth cervical vertebra inferiorly through the mediastinum to about the level of the space between the fourth and fifth thoracic vertebrae. The last tracheal cartilage is elongated and has a hooklike process, the carina, which extends posteriorly on its inferior surface. At the carina, the trachea divides, or bifurcates, into two lesser tubes—the primary bronchi. One of these bronchi enters the right lung, and the other enters the left lung.



The primary bronchi slant obliquely inferiorly to their entrance into the lungs, where they branch out to form the right and left bronchial branches (Fig. 10-3, B). The right primary bronchus is shorter, wider, and more vertical than the left primary bronchus. Because of the more vertical position and greater diameter of the right main bronchus, foreign bodies entering the trachea are more likely to pass into the right bronchus than the left bronchus.


After entering the lung, each primary bronchus divides, sending branches to each lobe of the lung: three to the right lung and two to the left lung. These secondary bronchi divide further and decrease in caliber. The bronchi continue dividing into tertiary bronchi, then to smaller bronchioles, and end in minute tubes called the terminal bronchioles (see Fig. 10-3). The extensive branching of the trachea is commonly referred to as the bronchial tree because it resembles a tree trunk (see box).





ALVEOLI


The terminal bronchioles communicate with alveolar ducts. Each duct ends in several alveolar sacs. The walls of the alveolar sacs are lined with alveoli (see Fig. 10-3, A). Each lung contains millions of alveoli. Oxygen and carbon dioxide are exchanged by diffusion within the walls of the alveoli.



LUNGS


The lungs are the organs of respiration (Fig. 10-4). They are the mechanism for introducing oxygen into the blood and removing carbon dioxide from the blood. The lungs are composed of a light, spongy, highly elastic substance, the parenchyma, and they are covered by a layer of serous membrane. Each lung presents a rounded apex that reaches above the level of the clavicles into the root of the neck and a broad base that, resting on the obliquely placed diaphragm, reaches lower in back and at the sides than in front. The right lung is about 1 inch (2.5 cm) shorter than the left lung because of the large space occupied by the liver, and it is broader than the left lung because of the position of the heart. The lateral surface of each lung conforms with the shape of the chest wall. The inferior surface of the lung is concave, fitting over the diaphragm, and the lateral margins are thin. During respiration, the lungs move inferiorly for inspiration and superiorly for expiration (Fig. 10-5). During inspiration, the lateral margins descend into the deep recesses of the parietal pleura. In radiology, this recess is called the costophrenic angle (see Fig. 10-5, B). The mediastinal surface is concave with a depression, called the hilum, that accommodates the bronchi, pulmonary blood vessels, lymph vessels, and nerves. The inferior mediastinal surface of the left lung contains a concavity called the cardiac notch. This notch conforms to the shape of the heart.




Each lung is enclosed in a double-walled, serous membrane sac called the pleura (see Fig. 10-3, A). The inner layer of the pleural sac, called the visceral pleura, closely adheres to the surface of the lung, extends into the interlobar fissures, and is contiguous with the outer layer at the hilum. The outer layer, called the parietal pleura, lines the wall of the thoracic cavity occupied by the lung and closely adheres to the upper surface of the diaphragm. The two layers are moistened by serous fluid so that they move easily on each other. The serous fluid prevents friction between the lungs and chest walls during respiration. The space between the two pleural walls is called the pleural cavity. Although the space is termed a cavity, the layers are actually in close contact.


Each lung is divided into lobes by deep fissures. The fissures lie in an oblique plane inferiorly and anteriorly from above, so that the lobes overlap each other in the AP direction. The oblique fissures divide the lungs into superior and inferior lobes. The superior lobes lie above and are anterior to the inferior lobes. The right superior lobe is divided further by a horizontal fissure, creating a right middle lobe (see Fig. 10-4). The left lung has no horizontal fissure and no middle lobe. The portion of the left lobe that corresponds in position to the right middle lobe is called the lingula. The lingula is a tongue-shaped process on the anteromedial border of the left lung. It fills the space between the chest wall and the heart.


Each of the five lobes divides into bronchopulmonary segments and subdivides into smaller units called primary lobules. The primary lobule is the anatomic unit of lung structure and consists of a terminal bronchiole with its expanded alveolar duct and alveolar sac.



Mediastinum


The mediastinum is the area of the thorax bounded by the sternum anteriorly, the spine posteriorly, and the lungs laterally (Fig. 10-6). The structures associated with the mediastinum are as follows:




The esophagus is the part of the digestive canal that connects the pharynx with the stomach. It is a narrow, musculomembranous tube about 9 inches (23 cm) in length. Following the curves of the vertebral column, the esophagus descends through the posterior part of the mediastinum and then runs anteriorly to pass through the esophageal hiatus of the diaphragm.


The esophagus lies just in front of the vertebral column, with its anterior surface in close relation to the trachea, aortic arch, and heart. This makes the esophagus valuable in certain heart examinations. When the esophagus is filled with barium sulfate, the posterior border of the heart and aorta are outlined well in lateral and oblique projections (Fig. 10-7). Frontal, oblique, and lateral images are often used in examinations of the esophagus. Radiography of the esophagus is discussed later in this chapter.



The thymus gland is the primary control organ of the lymphatic system. It is responsible for producing the hormone thymosin, which plays a crucial role in the development and maturation of the immune system. The thymus consists of two pyramid-shaped lobes that lie in the lower neck and superior mediastinum, anterior to the trachea and great vessels of the heart and posterior to the manubrium. The thymus reaches its maximum size at puberty and then gradually undergoes atrophy until it almost disappears (Fig. 10-8).



In older individuals, lymphatic tissue is replaced by fat. At its maximum development, the thymus rests on the pericardium and reaches as high as the thyroid gland. When the thymus is enlarged in infants and young children, it can press on the retrothymic organs, displacing them posteriorly and causing respiratory disturbances. A radiographic examination may be made in the AP and lateral projections. For optimal image contrast, exposures should be made at the end of full inspiration.






SUMMARY OF PATHOLOGY



























































































Condition Definition
Aspiration/foreign body Inspiration of a foreign material into the airway
Atelectasis Collapse of all or part of the lung
Bronchiectasis Chronic dilation of the bronchi and bronchioles associated with secondary infection
Bronchitis Inflammation of the bronchi
Chronic obstructive pulmonary disease Chronic condition of persistent obstruction of bronchial airflow
Cystic fibrosis Disorder associated with widespread dysfunction of the exocrine glands, abnormal secretion of sweat and saliva, and accumulation of thick mucus in the lungs
Emphysema Destructive and obstructive airway changes leading to an increased volume of air in the lungs
Epiglottitis Inflammation of the epiglottis
Fungal disease Inflammation of the lung caused by a fungal organism
 Histoplasmosis Infection caused by the yeastlike organism Histoplasma capsulatum
Granulomatous disease Condition of the lung marked by formation of granulomas
 Sarcoidosis Condition of unknown origin often associated with pulmonary fibrosis
 Tuberculosis Chronic infection of the lung caused by the tubercle bacillus
Hyaline membrane disease or respiratory distress syndrome Underaeration of the lungs caused by lack of surfactant
Metastases Transfer of a cancerous lesion from one area to another
Pleural effusion Collection of fluid in the pleural cavity
Pneumoconiosis Lung diseases resulting from inhalation of industrial substances
 Anthracosis or coal miner’s lung or black lung Inflammation caused by inhalation of coal dust (anthracite)
 Asbestosis Inflammation caused by inhalation of asbestos
 Silicosis Inflammation caused by inhalation of silicon dioxide
Pneumonia Acute infection in the lung parenchyma
 Aspiration Pneumonia caused by aspiration of foreign particles
 Interstitial or viral or pneumonitis Pneumonia caused by a virus and involving the alveolar walls and interstitial structures
 Lobar or bacterial Pneumonia involving the alveoli of an entire lobe without involving the bronchi
 Lobular or bronchopneumonia Pneumonia involving the bronchi and scattered throughout the lung
Pneumothorax Accumulation of air in the pleural cavity resulting in collapse of the lung
Pulmonary edema Replacement of air with fluid in the lung interstitium and alveoli
Tumor New tissue growth where cell proliferation is uncontrolled





General Positioning Considerations


For radiography of the heart and lungs, the patient is placed in an upright position whenever possible to prevent engorgement of the pulmonary vessels and to allow gravity to depress the diaphragm. Of equal importance, the upright position shows air and fluid levels. In the recumbent position, gravitational force causes the abdominal viscera and diaphragm to move superiorly; it compresses the thoracic viscera, which prevents full expansion of the lungs. Although the difference in diaphragm movement is not great in hyposthenic individuals, it is marked in hypersthenic individuals. Figs. 10-10 and 10-11 illustrate the effect of body position in the same patient. The left lateral chest position (Fig. 10-12) is most commonly employed because it places the heart closer to the IR, resulting in a less magnified heart image. Left and right lateral chest images are compared in Figs. 10-12 and 10-13.






A slight amount of rotation from the PA or lateral projections causes considerable distortion of the heart shadow. To prevent this distortion, the body must be carefully positioned and immobilized.






Breathing Instructions


During normal inspiration, the costal muscles pull the anterior ribs superiorly and laterally, the shoulders rise, and the thorax expands from front to back and from side to side. These changes in the height and AP dimension of the thorax must be considered when positioning the patient.


Deep inspiration causes the diaphragm to move inferiorly, resulting in elongation of the heart. Radiographs of the heart should be obtained at the end of normal inspiration to prevent distortion. More air is inhaled during the second breath (and without strain) than during the first breath.


When pneumothorax (gas or air in the pleural cavity) is suspected, one exposure is often made at the end of full inspiration and another at the end of full expiration to show small amounts of free air in the pleural cavity that might be obscured on the inspiration exposure (Figs. 10-16 and 10-17). Inspiration and expiration radiographs are also used to show the movement of the diaphragm, the occasional presence of a foreign body, and atelectasis (absence of air).





Technical Procedure


The projections required to show the thoracic viscera adequately are usually requested by the attending physician and are determined by the clinical history of the patient. The PA projection of the chest is the most common projection and is used in all lung and heart examinations. Right and left oblique and lateral projections are also employed as required to supplement the PA projection. It is often necessary to improvise variations of the basic positions to project a localized area free of superimposed structures.


The exposure factors and accessories employed in examining the thoracic viscera depend on the radiographic characteristics of the individual patient’s pathologic condition. Normally, chest radiography uses a high kilovolt (peak) (kVp) to penetrate and show all thoracic anatomy on the radiograph. The kVp can be lowered if exposures are made without a grid.


If the selected kVp is too low, the radiographic contrast may be too high, resulting in few shades of gray. The lung fields may appear properly penetrated on such a radiograph, but the mediastinum appears underexposed. If the selected kVp is too high, the contrast may be too low, and the finer lung markings are not shown. Adequate kVp penetrates the mediastinum and shows a faint shadow of the spine. Whenever possible, a minimum source-to-IR distance (SID) of 72 inches (183 cm) should be used to minimize magnification of the heart and to obtain greater recorded detail of the delicate lung structures (Fig. 10-18). A 120-inch (305-cm) SID is commonly used in radiography of the chest.



A grid technique is recommended for opaque areas within the lung fields and to show the lung structure through thickened pleural membranes (Figs. 10-19 and 10-20). This technique produces an image with higher contrast.






Radiation Protection


Protection of the patient from unnecessary radiation is the professional responsibility of the radiographer (see Chapter 1 for specific guidelines). In this chapter, the Shield gonads statement indicates that the patient is to be protected from unnecessary radiation by restricting the radiation beam using proper collimation. In addition, the placement of lead shielding between the gonads and the radiation source is appropriate when the clinical objectives of the examination are not compromised. An example of a properly placed lead shield is shown in Fig. 10-25.



Mar 4, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on THORACIC VISCERA
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